Conventionally, we feel that a child should look exactly like his parents since he inherits 50% of his genetic material from father and 50% from his mother. The siblings, too, should look alike. But the reality is that, you the child may look quite like his parents, somewhat similar, or totally different. The same holds true of siblings (except for monozygotic/ identical twins). This all happens, or depends on the mixing of genes, that is passed on to the child in case of sexual reproduction.
Now, before we move further we should know what genes are?
Genes are small parts of DNA that encode for a specific protein. (These Proteins carry out specific function as was described in the previous blogs). Genes are present on structures called Chromosomes which are present in the nucleus of the cell. There maybe 25,000 to 35,000 genes in a single human cell. These Genes make up the hereditary material where each gene is responsible for the expression of a particular trait. Genes are present in pairs, one each on the corresponding homologous chromosome pair. From the pair, one gene comes from the mother and the other from the father.
From this knowledge, an obvious question comes to our mind- what are chromosomes?
A chromosome is a thread like structure or a strand which bears the genes. It is composed of a single DNA double helix molecule (the double helix is held together by complimentary base pairs as was described in one of the previous blogs). Instead of having one long piece of DNA, our DNA is broken into 23 pairs of shorter pieces called chromosomes. Of the 23 pairs, 22 pairs of chromosomes look the same in both males and females and they are known as AUTOSOMES . The 23rd pair differs in male and female and so is known as the Sex Chromosome pair. The Sex Chromosome pair is XX in a female and XY in a male. In females, one X chromosome is inherited from the mother and the other X chromosome is inherited from the father. In males, the X chromosome is inherited from the mother and the Y chromosome is inherited from the father.
Including The Sex Chromosome Pair (circled) (either XX or XY will be present)
(Photo credit- Shutterstock)
The Chromosomes including the sex chromosomes are present in all the cells, be it somatic or reproductive cell.
How the sex cells are different from the other cells of the body in the type of cell division methods?
The somatic cells other than the sex cells undergo a cell division called MITOSIS (equational cell division) by which new cells are formed (each daughter cell receives a full set of chromosomes from the parent cell ie. diploid condition) from the old cells which have a fixed lifespan. Due to Mitosis, damaged body cells are constantly repaired and replaced. So, in Mitosis, the chromosome number is maintained at 46.
The sex sells undergo another type of cell division called MEIOSIS (reductional cell division) apart from Mitosis. In males, MEIOSIS I followed by MEIOSIS II, occurs at the time of Puberty and as a result of this, 4 haploid Sperms (having half the number of chromosomes from parents cell) are produced from each germ cell.
In females, MEIOSIS I occurs during the embryonic stage, during which a fixed number of germs cells are produced. MEIOSIS II occurs on maturation, when 1 haploid Egg (and 3 Polar bodies which are discarded) is released during each Menstrual cycle.
What are homologous chromosomes?
- Homologous chromosomes occur in pairs.
- A pair of homologous chromosome contains one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization (ie. zygote formation).
- A pair of homologous chromosomes carry the same genes, one from each parent.
- The two X chromosomes are considered homologous, whereas the X and Y chromosomes are not considered homologous.
- The following diagram explains well about the homologous chromosomes and the sister chromatids :-
(Photo credit- Shutterstock)
What are non- sister chromatids?
Following the DNA replication, now each chromosome is composed of two DNA molecules. The two identical copies of chromosomes are called chromatids. So, now each homologous chromosomes pair contains four chromatids.
Non-sister chromatids are the chromatids of the homologous chromosomes pair.
Where does mixing of genes occur?
Mixing of Genes occurs in Prophase I Stage of MEIOSIS I in both males and females through a process of Crossing Over and Recombination between non-sister chromatids of homologous chromosomes.
How Crossing Over occurs?
- Crossing over is basically a phenomenon of breaking, transposition and fusion of chromosomal segments.
- The non-sister chromatids first break at certain points due to the activity of enzymes called endonucleases.
- Then the broken segments get exchanged and finally get fused with the other chromatid due to the action of the enzyme called ligase.
- A little amount (3%) of DNA synthesis occurs during the time of crossing over to repair the broken chromosomes.
- The resultant cross after crossing over is called Chiasma or Chiasmata.
(Photo credit- Shutterstock)
Science-FUN-damentals 
Good information about complex mechanism.